This invention relates to a device for controlling the flow of coolant through an assembly in a nuclear reactor. More particularly, this invention relates to a device for controlling the flow of coolant through an assembly in a nuclear reactor which device responds automatically to changes in temperature of the flowing coolant.
A nuclear reactor comprises a core which holds in a matrix pattern a variety of assemblies such as fuel assemblies, blanket assemblies, control rod assemblies, and reflector assemblies. The matrix pattern in which the assemblies are arranged facilitates a sustained nuclear chain reaction.
In a liquid metal fast breeder reactor (LMFBR) each assembly is housed within a hexagonal tube about 200 inches long and about 6 inches in diameter. Sodium coolant enters each assembly through a bottom adapter, flows upwardly past the heat producing elements of the assembly, and exits the assembly in its heated condition via an upper adapter. The coolant then flows out of the vessel, carrying the heat to a heat exchange system where it is converted to electricity.
In order to sustain the nuclear chain reaction about 600-1000 assemblies are required. Each type of assembly has a different rate of heat production; this rate is further affected by the location of the assembly in the matrix pattern.
It is desirable to have the coolant leave each of the assemblies at about the same temperature to reduce severe thermal stresses on the proximate components of the reactor. Accordingly, each type of assembly is appropriately orificed at the bottom adapter and often at the reactor core support plenum. In this way the amount of coolant flowing through each assembly housing is controlled such that a greater volume of coolant flows through those assemblies which produce a larger amount of heat, and a lesser volume of coolant flows through those assemblies which produce a smaller amount of heat. This technique for providing a relatively uniform exit temperature is well known in the art of nuclear reactors.
Breeder reactors are unique in that new fissionable material is created in various portions of the reactor core, particularly in the blanket and reflector assemblies. The new fissionable material causes a significant increase in the localized reactivity and a corresponding increase in the localized heat production of these assemblies over their lifetimes, which is typically about 3-5 years. The volume of coolant flowing through these assemblies must be large enough to accommodate the maximum heat production which occurs at their end-of-life. This large volume of coolant typically results in overcooling of the assemblies at their beginning-of-life when their heat production is less. It would be desirable to be able to mechanically adjust the coolant flow through these assemblies over their lifetimes to accommodate changes in the rate of heat production. To date, it has not been possible to mechanically adjust coolant flow safely and economically.